If only it was so easy to give simple answers to the Rules of Thumb question . ..........................there are just so many variable to consider,........... big turbo with narrow map high pressure capability vs small turbo with wide map but limited pressure , diesel vs petrol turbo , sophistication, or lack off, with regards the fuel presentation , and/or type of fuel used , the general belief by guys that they know better so they cut corners with recommendations, the list goes on and on and on ....................I spent hundreds of hours trying to sort out engine problems for guys on the DIY Site before writing up the RoTs , and then Jesse wanted to construct Jetspecs to save guys having to do the calculations , there are a number of compromises built into Jetspecs , but we had to live with them .

You should know me by now, always trying to break limits and achieve what peoples say it's achievable ... Lollllllll

NIce Jetspecs by the way, and maybe one day you guys could make it so it can diffirenciate between few fuel types such as propane, kerosene (Diesel) or gas.

So this said, I will obviously stick to my numbers then and since they provide me great results, such as low rpm start-up requirement and low EGT, although I DO admit your hole spread design offer better results in terms od stability, EGT and maybe part of the reasons why a mid size DIY GT4708 Turbine get autonomous around 10,000 rpm to slowly spool-up to idle alone.

In fact, I don't know if you have noted in the above movie, but the turbine DOES get autonomous after only 10 seconds of starter time, and we did even shorter spool time when she's hot ...

So, I guess it all boiles down to "If you have problems with your DIY Turbine, just contact John .... Or Luc then ...

LOL.....Theres no need for Jetspecs anymore , lotsa Youtube videos showing how to make a jet engine from a turbocharger using "stolen" info from the DIY Site .....................heh heh , its a pity most of them won't work properly though because the guys making the videos don't understand the basics, they think all you need is a tin can fitted to a turbine scroll for it to work , they do "work??", but theres a big difference between making some noise and burning some fuel to actually having an engine that performs to its full potential.

At 10,000 rpm the GT4708 would have needed a delicate touch on the throttle to avoid thermal induced surge problems , the lack of "backpressure" on the engine would have greatly assisted the spoolup , your 450 C TOT indicates full pressure drop across the turbine stage , it'll be more difficult with a tight jetnozzle on the engine and those temperatures up another 100C degrees even at idling , and some 300 C degrees at full throttle,................
heres some "early" idling numbers for the 10/98 engine https://www.youtube.com/watch?v=lrLN7idx2V8 .

Remember , a cool running turbine engine isn't making any power , if you use GT6041 turbos with their Inco turb wheels you'll be developing the engine to produce TOTs of at least 800 C at full throttle , and if you use kero/diesel rather than "forgiving" propane , your combustion process will need to be right .

Yep , call Luc if you have problems with your DIY engine , I'm slowly retiring from the scene

OMG ... My P2 maybe runs around 2 psig. @ 40,000 rpm idle condition. In fact, the compressor pressure port is equiped with two pressure switches, one that kills the starter @ 1.5 psig. and one that provide overspeed protection, thus, killing the turbine if P2 exceeds 40 psig. which is around 80,000 rpm and NOWERE NEAR this GT4708R upper limit.

Bare in mind John, that this unit is destined to the industrial market and "Safety" is a most important item for IT IS BOUND to get in the hand of an idiot that will want to ear what 100,000 rpm sounds like. The fuel system is according to Canadian Standard (CSA), oil system is protected in both temperature and pressure, starter déactivation prevents anyone from re-initiating startup when this baby is turning 40,000 rpm, which would be catastrophous and overspeed is prevented.

As for "Heat", you and I both know that "Thrust" is established by exhaust gas delta "Pressure", "Velocity", "Temperature" and "Mass" in relation with the atmospher. But not wanting to see these babys fitted in the back of a shoping basket and workers trying to break 1/4 reccords, I greatly reduced the "Heat" and "Pressure" factors just so it suits the applications ... Not all these "Evil Kenivel" idots.

In fact, the unit's tail pipe exit as a 2.9" diameter, nothing to break sound barriers with ... But it's a pretty quiet, easy to use and safe system, although I know I could Squeeze that tail pipe exit a bit more, which would crank up EGT and thrust ...

Hooo and by the way ... The system DOES have a "High Power" swith that from testing, we realized it was very usefull using it during start sequence, especially for starts under very cold conditions ... A bit like a "Choke" button ... This said, I can already hear you say "I KNEW IT ..." He he he (Ref. : your comment : "At 10,000 rpm the GT4708 would have needed a delicate touch on the throttle to avoid thermal induced surge problems") ...

No ... Seriously ... I have a bit of a problem here John and i would like to have your opinion on something I never tried and I'm about too ...

The attached file shows my machine's exhaust pipe which basically consist of the turbine exhaut pipe shooting into a bigger pipe, this bigger pipe heretoo shooting into a large pipe. Basically, we have a 2 stage "Ejector" here, and his purpose is to EJECT the turbine's cabinet heat while reducing exhaust gas temperature for the application. So, the area or bracket marked "Air Ejection #1 (Cabinet Air)" is the first ejection stage, and the area marked "Air Ejection #2 (Exterior air)" is the second ejection stage ... Thus, the result from those 2 ejection stage process is a much cooler "Exhaust Gas & Cold Air Mix" at the outlet while extracting any heat from the cabinet that might be generated by the turbine.

Now, my problem is that once connected to the application using a flexible hose, I get not only a bit of spitting from the "Air Ejection #2" are, but also a bit of heat "Flowing Backward" into the cabinet through the "Air Ejection #1" area. Although the "Spitting" from "Air Ejection #2" is of no consequences other then just loose a bit of effeciency, the "Backflow" into the cabinet is a NO NO for it will overheat the turbine after a while.

From my concept and in order to reduce the system overall length, you will note the turbine's tail pipe exit is right before the 55° elbow at the area marked "Exhaust Gas Expansion Area", and this is where I beleive I have a problem. As the gases exit the tail pipe, there is no wall on which to make contact to acheive a most efficient ejection process as the elbow upper wall going up tence to stay away from the expanding exhaust gases, and therefore, the ejection "Pumping effect" at this location is very weak. In fact, this is probably why temperature recorded at the top portion of the ejector tube near the cabinet wall is 125°F and increasing while the bottom remains at 90°F and stable.

So, my questions are simple;

1 - Can I reduce the gas turbine tail pipe in order to allow full exhaust gase expansion and full contact with the ejector tube's wall BEFORE it reaches the elbow?
2 - My turbine's tail pipe is 16.06" long, something that I got from a "Rule of thumb" such as "X Time the turbine exducer diameter" (Can't remember from where since I have PILES of notes), does it have to be that long?
3 - Although the diameter and exit builds pressure, is the length REALLY important?
4 - Although I don't want to raise the EGT for application purposes, do I get your blessing for my 2.9" diameter or you have a better number?

Checkout the length of jet nozzle I used on my GT6041 http://jetandturbineowners.proboards.co ... art?page=8 , I used a central " boss" tube and straightening vanes to produce a smooth laminar flow into the jet nozzle , the gases exiting a turbo exducer fed from a scroll are a "dogs breakfast" of speeds and angles , long jetpipes only waste energy from wall friction , the ID of the GT60 jetpipe is the same diameter as the turbine wheels exducer , no step bigger or smaller , just straight out of the turbine , any swirl in the gases is straightened by the vanes and its into the nozzle and expanded all within 150mm, the fact that the central tube extends to the exit plane of the jet nozzle means there is less reduction in the jetpipe to nozzle diameter and less disturbance to the flow , the GT60 jetnozzle was just a rough nozzle to put backpressure on the engine , a proper one would have had a "bullet" past the end of the jetnozzle plane

Your ejector tubes could do with a bellmouth inlet .

The clearance between the long jetpipe and the first ejector tube is a bit narrow , I'd imagine the flow will be thermally choked very quickly .

Is there any reason for having two ejectors rather than a single larger one exhausting the cabinet ??

The fact that your jet nozzle is blasting against a bend isn't good , your 2.9" nozzle produces a velocity of ~950ft/sec , my calcs used a PR of 3:1 , 78% comp effic,a 2.25 PR across the turbine stage resulting in a total pressure in the jetpipe of ~4 psi ...........at 950 ft/sec that jet of gases is "hard" , it will slam into the bend and cause all sorts of problems

My suggestion would be a jetpipe as short as possible with a bend , the lower gas speeds in the "large radius" jetpipe bend won't be such a problem , once around the bend the jetnozzle will exhaust into a long straight section where it will "eject" more efficiently as there will be a more laminar flow rather than the mass of turbulence you currently have at the bend , I'd be inclined to have all of the ejection air fed from the cabinet (unless theres a strong reason not to ,....... safety ??) through a larger diameter tube at least the size of your second ejector, as there'd be less chance of thermal choking a small gap if two ejectors are used .

The one "problem" with the GT4708 is the turbine efficiency at a 69% max , this is causing you to run hotter temperatures than you would need to than if it was up at 80% or above as can be the case with a scroll with NGVs , the comp efficiency is excellent , the turbine stage lets it down , with lower operating temps there'd be less need for ejector air .

Before I begin, let me underline how such a GREAT guy you are ... No, seriously, .... I have wintness you help shitload of peoples here, young and old, newbies and "Less nembies" (Not to say experts) ... You're such a great person and life has a special place for you buddy ... It will come ..

Okey ... Now back to our chickens, and to answer your question about why there are 2 ejector stages, the answer is "Temperature reduction" ... That's all ... Well dynamically speaking. There is another factor that calls for that second ejector, and it's the "Flexible Hose" that attach to this second ejector tube.

Okeyy ... Here's a bit of additional info for you. This system is a "Heat Generation" unit that uses a 13 foot long "High Temperature Flexible Hose" to bring that heat where it's needed. Now, not only this hose is limited to 600°F, but it also comes in standard diameters such as 2", 3", 4", 5" ... ect ect diameters. Since that turbine's exhaust pipe is already 3.3" ID, using a 4" hose connected to a 4" ejector tube only provide us with 1 ejection stage and the temperature was found to get to high. Using a 5" hose connected to a "Single" 5" ejector tube to again only use 1 ejection stage, then, the diameter diference between the turbine tail pipe (3.3") and the 5" ejector tube is so great, that we would have to extend that "Single" ejector tube longer so exhaust gases can expand to create "Pumping", but again, there was no waranty the "One Stage Ejection" would provide enough hot gases dillution to drop that temperature below the 600°F mark, not mentioning the overall system would be longer then necessary.

So, this is why we went from 3.3" tail pipe to a 4" first ejector tube to a 5" second ejector tube to a 5" hose, so we could keep that hose as small as possible while dropping the temperature to that 600°F mark, and why we went with two ejection stages for "Overall" considerations (i.e. : Hose temperature limitation, system overall length, Flex Hose Standard size, compactness ... Bla bla bla).

Now, your message above kinda made use realize that wanting to stick to "Standard Size" even with the first ejection stage tube although this one IS 100% "Fabricated", was a mistake. In fact, although I always knew that when using ejectors, "More you get away from the source, smaller the Gaps must be", I obviously "Drop" (Forgot) this last one and it came back BITTING me hard ... ((*(%/?%$%%$(&?%$%$/? ... stupid me).

Starting from the turbine's tail pipe which is 3.3", I should have gone maybe 4.5" for the first stage ejector and 5" for the second. And the worst here, is that I have a CFD Analysist software that would have told me better if I would have gone through the effort of using it ... Like I JUST DID (See the attached Pictures) ... Lollllllllll

I have to admit though, that one of the reason we were never tempted to CFD the rear end of our system, is the fact that we can not have data from that flex hose such as resistance, or roughness or back pressure build-up data ... Nothing ... NADA ... So we had no choice of venturing on a "Trial & Error" design using that hose.

Anywayyyyy ... The CFD is done now, thanks for your inputs, and the system is beeig corrected as I'm writing you this message, including dropping the elbow and overall length reduction.

By the way, I have a new SUPER computer here that can run complex thermodynamic analysis that use to take 50 hours +, now in 20 minutes. So, if you ever neeed complex CFD for your Babys ... Just say so and it's free for you Buddy ...

LOL.....you're such a flatterer ................heh heh , you should know I don't have an ego , flattery is wasted on me .

Yep , making sense why you need those 2 ejectors now .

Laminar flow downstream of the nozzle will keep backpressures to a minimum , is there any way you can add a couple of turning vanes in that elbow , I still see that "hard" jet efflux hitting the elbow and wanting to go straight on , hitting the curve will cause turbulence and that turbulence will "fill" the flexible pipe allowing less room for ambient air to enter .

A jet efflux takes considerable time to pickup ambiant air , it requires ~90 jet nozzle diameters for the efflux to break up , but you don't want the large low frequency stuff , you only need the high frequency finer mixing to maintain some degree of laminar flow so your current design is looking good .

I did some experiments years ago with the exhaust of my TV84 turbo engine , it had a 76mm jet nozzle originally , I then made a "bunch of bananas" type exhaust with 7 small jetnozzles all housed in a 14 inch diameter tube ( ejector) , the distance between the 7 small nozzles exits and the end of the duct was ~10 diameters , it didn't entrain much ambient air though , .........but once additional length was added to the duct things changed considerably .

A jet efflux "expands??" at an ~4 degree included angle.

Don't forget to add a bellmouth on the entrance to those ejectors , remember the vena contracta , it''ll significantly reduce the airflow into your gap space as well as cause turbulence ,..........turbulence is your biggest enemy , smooth laminar flow will get those temps down further still , add straightening vanes wherever you can .

Nope I'm not a flaterer ... Just giving to Cesar what belongs to Cesar that's all.

As for the elbow, we droped it (abandonned) and are now carrying CFD of a "Straight" 2 stage ejector, changing sizes and length, and STRANGELY, it seems my narrow gap first stage and larger gap second stage are giving the best results (I'm puzzled here) over larger first stage gap and smaller second stage gap.

We did add inlet "Flairs" to each ejector inlets but now our biggest problem seems to be coming from a "Too efficient" turbine tail pipe shooting gas too fast through these stages like a God Damned Laser beam ... But we will figure a way to max out this ejection propcess.

The smaller first stage gap could be better due to the finer entrainment at that point not requiring a lot of air , downstream as the jet surface starts to get "rougher" the larger clearance will have the capacity to supply air into that rougher jet boundary layer .